We begin in boston tonight amid reports of a break in the case. And here's the question. Has the bomber been spotted on a surveillance tape? As we've heard there of the videotape everywhere at that moment on that day of the marathon. Experts have been scouring the images. And we also know that on this night 62 injured are still fighting for recovery in the hospital, and 12 of them remain in critical condition. So let's get straight to the latest news right now from abc's chief investigative correspondent, brian ross. Brian? Reporter: Diane, authorities put out word today, just 48 hours after the bombs went off, that they might have found the bomber. The developments came on a day of unfulfilled expectations and a serious case of security jitters. This is not a drill. The federal courthouse where any suspect would stand trial was evacuated late this afternoon because of a bomb threat that was, understandably, taken very seriously. What are they telling you? Code red evacuation. Code red evacuate the building. Reporter: No bomb was found. But the law enforcement response forced a postponement of a news conference where officials had planned to talk about a possible suspect. Authorities say the fbi is focused on photographs, including some taken by a surveillance camera at the lord & taylor store, that show a man in the race-day crowd, using a cellphone with a black bag strapped across his chest that may contain the bomb. The key frames are just a few of the millions of frames of video examined in the last 36 hours by a special team of fbi forensic experts, the operational technology division. They have set up facilities at a restricted location near the boston harbor. And they are masters at what is known as facial recognition, the new science of converting a face into a name, demonstrated here by an indiana company that was used by the fbi following the 9/11 attacks. In this investigation, there will be hundreds of thousands of images. If they have a suspect identified, they know the area that suspect was in, they'll be able to then track the person back to an image and say, here we have him. Reporter: The fbi tech agents are also examining tiny bits of debris recovered at the crime scene. There are wire fragments with manufacturing names, tiny nails used as shrapnel, as well as the mangled remains of the pressure cooker that agents say was used to build the bomb, and may well have fingerprints still on it. One of the keys in any trial is linking the evidence you have to the suspect. It's extremely important to match the bomb to the bad guy. Reporter: Ultimately, whoever is arrested will almost certainly face at least three counts of premeditated murder, for the deaths of 8-year-old martin richard, 29-year-old krystle campbell, and the third victim identified just today, boston university graduate student lingzi lu, a native of china. There may be as many as a thousand agents assigned to this case.

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